Engine Design RPM

[Mike Rometer]

I would have thought that dynamic balance would play a large part in the available RPM of a steam engine, as it does in I.C. engines. Also the ratio of bore and stroke. Short stroke I.C. engines normally rev much higher than long stroke. The subject is complex for many reasons.

[barts]

I would have thought that dynamic balance would play a large part in the available RPM of a steam engine, as it does in I.C. engines. Also the ratio of bore and stroke. Short stroke I.C. engines normally rev much higher than long stroke. The subject is complex for many reasons.

Unless you're trying to build a planing steam boat with a monotube or Lamont boiler, the weight of the engine is a small part of the power plant weight. Choosing a low rpm (500 rpm or lower) engine speed allows one to avoid the use of a reduction gear and still drive a
good size propeller, which helps efficiency.

At these kinds of speeds, sizing counterweights to offset piston and crosshead weights is sufficient... although I have considered counter-rotating balance shafts....

- Bart

[Stoomsnor]

Although you posted your question a few years ago, I currently had the same question in trying to get a large number of calculations in a speadsheet.
All becomes more and more clear to me now, I was wondering if you were able to salve the mistery of RPM?
It's still the one thing I am missing and I would like to hear from you!

[fredrosse]

As stated previously, engine RPM is determined based on the mechanical limitations of the engine, (mostly stresses associated with engine pressure forces and inertia loads), fluid flow limitations in getting the steam into and exhaust out of the cylinder, and by the requirements of the driven machinery.

If you want to delve into the actual mechanical engineering aspects of this subject, a good reference that actually covers the fundamentals is: "Internal Combustion Engines" by Lester Lichty, 1939, McGraw Hill Publishing. To use this very real reference, you will need to know the fundamentals of statics, dynamics, and inertia, mechanical strength of materials, structural design, fluid flow and thermodynamics. Most of the steam engine design books published earlier address the steam engine design considerations in a more empirical way, suggesting typical designs for typical applications, with simplified stress and balance instructions. Either way, it is not generally something that can be done with a few days of computations.

Could you tell us details of what you are actually trying to do here? The large amount of experience on this forum can often provide help, many of us have studied steam engine design for many years. I have several hundred hours spent developing spreadsheets for various aspects of steam engine design, probably more than 50 different EXCEL programs.

[Stoomsnor]

I think it's all very very interesting what you bring up, there is much more to it then I expected...
I am in the process of trying to understand steamengines after buying one myself, an incomplete twin Lentzcompoundengine.
A wonderfull engine and very professionaly build. I want to compleet it with the same precision an craftmanship as it was build, so I have to do a lot of homework... (I never liked homework upto now...)
After reading quit a lot of books from my father who used to be a mechanical engineer on oceangoing tankers I decided to put all the formulas in a spreadsheet.
One of the reasons for doing so was that I made very many mistakes with the calculations with ofcourse very strange results... Than I discovered that being able to make very fast calculations with the help of Excel I could see what changes ment for other things in the process. By doing so I am able to see how coherrent things are (sorry for my English).
Also excel becomes very handy in transforming British or US-data into metrical to make it more understandable for a simple Dutch guy...
So I sorted out a lot of things already. The only thing that puzzles me still is the rpm and how to deal with it.
In my understanding I have to do my mathematics backwards on this sublect.
For instance I want to build an engine for a ship. My theory in this case would be:
At wat speed can the hull be driven,
what size of propellor do I need with what rpm to be able to make that speed in an optimal way,
what enginepower do I need to make 80 procent of the hullspeed in an enduring way,
am I able to make that number of rpm or do I need a gearing,
do I build it for force (tugging), long distances, recreation or for the highest possible economical output,
do I choose for a fast running small engine or an easygoing big engine,
is there sufficient steampower from my boiler, etc,etc...
In my calculations I noticed rpm has a big part in this all, on the other hand the torque at low rpm of steamengines is astonishing...
Are the bearings able to coop with the forces,
will I be at risk drawing water from the boiler instead of steam...
I am very interested in Excelprograms, for instance steamtables including the superheated.
It is realy nice to see how helpfull my fellow-steamenthousiasts from all over the world (is that even a word?) are on this forum!!!
Frank

[barts]

I'm afraid all my references are American books.

I would recommend as reading material David Gerr's Propeller Handbook for basic boat speed calculations, propeller sizing and efficiency discussions.

For bearing loading, design details, etc. I'd use an older machine design text such as Shigley. For steam engine details, Kent's or Mark's Engineering Handbooks from the first part of the 20th century are excellent references, and cheaply available on EBay. However, these books do expect some aptitude and/or background on the part of the reader.

- Bart

[fredrosse]

In performing my employment in the steam power industry I have developed many EXCEL spreadsheets to help design and analysis of many systems and machines. I build these during the last 20 years during my "day job", and some of course deal with reciprocating engines, boilers, combustion, draft systems, and of course some marine analysys work. I have several that present in both Metric and English units. I do not know how to share some of these files with you, if you could tell me a method to do this.

As a starting point for developing EXCEL programs relating to our steam launches, look at the functions presented in the FAQ section of this forum.

The texts recommended by Bart are much better, but will require significantly more study.

[Stoomsnor]

Thanks for the answers guys!
I will contact you Fred, thanks for offering help!